Remove BN_MONT_CTX_init.
One less exported function. Nothing ever stack-allocates them, within BoringSSL
or in consumers. This avoids the slightly odd mechanism where BN_MONT_CTX_free
might or might not free the BN_MONT_CTX itself based on a flag.
(This is also consistent with OpenSSL 1.1.x which does away with the _init
variants of both this and BIGNUM so it shouldn't be a compatibility concern
long-term either.)
Change-Id: Id885ae35a26f75686cc68a8aa971e2ea6767ba88
Reviewed-on: https://boringssl-review.googlesource.com/6350
Reviewed-by: Adam Langley <alangley@gmail.com>